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System Effects and
Stacks
BA-5-3 2
System Effects
A System Effect Factor is added to the calculated system resistance and the fan is selected for a higher pressure point.
The fan will operate at the originally calculated system resistance.
“Industrial Ventilation” and “Fans and Systems” AMCA Publication 201 contain additional information on System Effects.
Non-uniform flow into a fan inlet by a 90º round section elbow – no turning vanes (Fig. 7-23)
Excerpted from the Industrial Ventilation Manual, 23rd Edition
Example of a forced inlet vortex – spin or swirl (Fig. 7-24).
Controlled diffusion and establishment of a uniform velocity profile in a straight length of outlet duct (Fig. 7-18).
Outlet Duct Elbows (Fig. 7-20)
Position C, No duct, 1.20VP
System Effect – Elbow at Inlet
Elbow with no straight duct R/D = 2.0
From Figure 7-23
System effect of R-S
From Figure 7-26
“R” = 1.2
“S” = 0.8
Fsys = (0.8 + 1.2)/2 = 1.0 VP
Restricted Inlet
22” duct = 2.6398 sqft
26” fan inlet = 3.687 sqft
Restricted Area = 2.6398/3.687
= 0.716
From Figs. 7-25 & 7-26
System Effect = 0.8VP
System Effects - Fan Outlet
No duct on fan outlet
Figure 7-18
Blast Area/Outlet Area = 0.9
SEF = “V-W”
From Figure 7-26
“V” = 0.26
“W” = 0.18
Fsys= (0.18 + 0.26)/2 = 0.22VP
Total System Effect
On inlet – 1.0VP
On outlet – 0.22VP
Restricted Inlet = 0.8VP
Total = 2.02VP
If V = 3500 fpm, VP = 0.76”wg
System Effect = 1.54”wg
Review
System effect is the estimated loss in fan performance from non-uniform airflow
System Effect Factor (SEF) is used to determine a correction value, in inches of water gauge, to be added to the system pressure losses and are represented in terms of VP
System effects are in addition to the system losses due to friction
How Can We Eliminate System
Effects?
Relocate the fan to eliminate elbow at inlet
Resize inlet duct to eliminate restriction
Add a no loss stack with a transition
VP will be lower due to larger inlet duct
Eliminating the System Effects
Exhaust Stacks
Do not use weather caps! – Losses associated with them
– May also have other system effects
No-loss stacks – Stack diameter = D + 1”
– Stack height = 4D + 6”
– D = duct diameter
Maintain discharge velocity
of > 3,000 fpm
Weather Cap
No-Loss Stack
D
Stack Height Above a Building
c
Discharge Stacks
• Exhaust beyond building envelope to avoid recirculation into air intakes • Provide sufficient dispersion
Proper Stack Design
Stack velocity should be 1.5 times wind velocity to prevent downwash
A good stack velocity is 3000 fpm, prevents downwash up to 2000 fpm (22mph)
High exit velocity poor substitute for stack height
Proper Stack Design
Stack velocity above 2600 fpm should prevent rain from entering stack while running (Rain Vt approx 2000 fpm)
Locate stacks on highest roof when possible
Don’t use rain caps
Separate exhaust points from intakes
Modeling
Simple one in ACGIH Vent Manual
More complex computer models
–Simplest
–Most complex
Do any give the real answer?
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